The present invention relates to Environmental Control Systems (ECSs) and, more specifically, to an improved ECS and an improved method of conditioning water vapor compressed air while assisting water condensation with a vapor cycle system (VCS).
ECSs are used to provide a supply of conditioned air to an enclosure, such as an aircraft cabin and cockpit. Typical air cycle ECSs operate on a flow of high energy bleed air taken from an intermediate or high pressure stage within a jet engine having multi-compression stages. The bleed air is typically pre-cooled within a primary heat exchanger with heat being dumped to ambient air, and then flowed to a turbine. The air expands in the turbine, providing cooling as well as mechanical energy to drive a fan drawing ambient air. A compressor may be provided in addition to the simple cycle above to create a "bootstrap" cycle, whereby the compressor is used to increase the air pressure prior to expansion in the turbine. In that case, a secondary heat exchanger removes the heat of compression before the air is admitted to the turbine. The power for the compressor is also provided by the turbine. Air from the turbine is discharged to the cabin to provide cooling. Additionally, but essential to the design of ECSs, specific equipment and cycle configuration are employed to condense and remove most of the incoming moisture so that relatively dry air is supplied to the cabin and internal freezing of the system components is avoided.
Current state of the art ECSs have been referred to as 3 wheel and 4 wheel high pressure water separation systems. A reheater heat exchanger is typically used to pre-cool the humidified air with subsequent condensation primarily occurring in a condenser heat exchanger, with this process occurring prior to expansion in the turbine and, therefore, being at "high" pressure. Thereafter, the condensed water is extracted to produce dehumidified air. Those systems use the dehumidified air, after it has been cooled during expansion through a turbine, as the heat sink or coolant medium in a heat exchange process to cause the condensation of the humidified air in the condenser.
In current ECSs, condensing water from vapor to liquid form requires a considerable amount of energy removal from the air/water mixture, and that energy is transferred in the form of added heat to the coolant air stream in the condenser. In the 3 wheel system, the added heat is directly passed onto the ECS delivery stream, resulting in a warmer supply to the cabin. To offset the result of a warmer supply, a larger input of pressure energy to further cool the air in the turbine and/or larger heat exchangers may be required. In the 4 wheel system, a second turbine wheel is added and "recovers" part (up to 20%) of the added heat energy, thus obtaining a more efficient cooling process.
Vapor cycle ECSs (VCSs) have also been used for cooling enclosures, in particular when large heat loads are present with no need for or availability of large pressurized airflow that could be used to power an air cycle. Compared to an air cycle, a VCS has a higher coefficient of performance--that is, the ratio of cooling capacity achieved to the energy input required to power the system. VCSs operate on the principle of compressing a working fluid in the vapor state and cooling it by heat transfer with a suitable heat sink medium (such as ambient air) so the fluid changes phase and becomes liquid. Expanding the working fluid absorbs a considerable amount of latent heat and thereby cools the desired enclosure indirectly by heat transfer during the evaporating process in an evaporator. In the evaporator, the working fluid is interfaced with air or a transport fluid that later circulates through the cabin.
While the above prior ECSs have utilized two air streams for heat exchange, some have sought to integrate a VCS with an air cycle system. Such hybrid systems have the potential to combine some of the advantages of air and vapor cycles for the purpose of more efficient cooling, water condensation and extraction. But in those prior hybrid cycles, vapor cycle systems have not usually been used to condense water vapor for subsequent dehumidification. Rather, the VCS has been commonly used to further cool air stream branches coming off the main air stream, with the branches acting as dedicated cooling mechanisms for electronics or other localized loads. As an example, the VCS disclosed in U.S. Pat. No. 4,966,005 is not used as an aid to condense water. Instead, the VCS is used to cool the air stream after it has been expanded and then heated in a condenser. U.S. Pat. No. 4,963,174 does disclose a VCS to cause condensation in the air stream prior to expansion in a turbine. But a disadvantage to such system is that the VCS condenser rejects heat exclusively to ambient air and, therefore, the system receives no benefit from recovery of the heat of condensation.
Despite the advantages in prior ECS designs using an air cycle, vapor cycle or hybrid cycle, the designs still have limitations, including those relating to energy loss in the water removal process. As water is condensed, considerable energy is expanded, and there is a need to recover the largest portion of that energy in the form of cooling, rather than wasting it by rejection to ambient air or increasing the supply temperature. Water condensation energy provided by a cooling turbine in an air cycle system is added (in whole or in most part) in the form of heat to the air supplied to the cabin. Conversely, water condensation energy provided by a VCS which rejects it to RAM air does not benefit the thermodynamic cycle and represents an unrecovered loss. Also, ECSs based on an air cycle have an inherently lower efficiency, or coefficient of performance, than VCS cycles. An increase in the coefficient of performance or efficiency resulting from the addition of a VCS means the ECS could provide more cooling capacity for a given ECS unit size. Alternatively, greater efficiency can mean a smaller sized ECS unit for a given load.
As can be seen, there is a need for an improved ECS and an improved method for high pressure water extraction using the combination of an air cycle and vapor cycle which will increase efficiency. Also, there is a need for an ECS that can utilize a VCS for assistance in water condensation while providing for recovery of the heat of condensation in the form of energy that would enhance the cooling capacity of the system.